Collectors often call the support for any single leaf a "stem" Although I doubt this post will change the use of the term, the petiole is actually the part of the plant that supports any leaf and is what most people call a "stem". The stem is the plant's base or axis and is the plant's support. The stem is not the stalk that supports any single leaf. The stalk that connects the leaf blade to the stem of any plant,not just an aroid, is a petiole. Some internet definitions don't make the distinction clear between a stem and the petiole since the petiole is truly a part of the leaf structure. I asked my friend botanist Dr. Tom Croat to explain, "A leaf consists of a petiole and a blade. A petiole may also have a sheath and a geniculum. As a further twist the geniculum actually extends briefly onto the blade at times so is structurally a part of both the blade and the petiole even though we consider it to be technically a part of the petiole." When you read statements on sites such as Wikipedia that state "A stem is one of two main structural axes of a vascular plant" the article is talking about the entire leaf unit, not just the leaf blade. The stem and petiole aren't one and the same. The second main structural axis mentioned is the root system. The petiole may take on many shapes including round (terete), "U" or "C" shaped (sulcate), "D" shaped and may even have odd shapes such as being quadrangular or hexagonal. The petiole may also have patterns including lines (striate) or grooves (also known as sulcate) up and down the petiole's axis. They may also be very hard or somewhat spongy to the touch. Botanists use the shape and features of the petiole to*help determine the species of the plant. The stem found at the base of the plant produces buds, nodes and internodes. In climbing species the stem is also found clinging to the supporting tree held in place by the roots. Its purpose of the stem is to collect and store water and nutrients absorbed by the roots. The water and nutrients are then distributed through the plant by the petioles. The buds and roots grow from the nodes and then the buds produce the petioles. In many rain forest species those roots never touch the soil since many species grow suspended on the trunk or limb of a tree! The blades then convert carbon dioxide in the air into oxygen for other living organisms such as rain forest animals to breathe. A geniculum is a part of the petiole not found on all plants. The geniculum is common on plants such as an Anthurium and allows the leaf blade to rotate to orient itself to the light. The use of the word "stem" to indicate the support for a leaf apparently began as a result of the beautiful rose. When we buy a dozen roses we find the flower at the end of a stem. In the case of the rose that support actually is a stem since it is a non-woody shoot of the true stem which is likely climbing some support. By the way, the "thorns" aren't true thorns but instead are modified stems. When we pick up a rose in a florist shop the leaves have almost always been removed so we forget those leaves also had a support. That support was a petiole. The flower is a terminal flower on the end of a stem shoot which is very different from the support for any leaf. So the rose flower is on the end of the stem but the leaves grow on their own petioles. I'm sure the term will stay in common usage but at least you'll know the support of any leaf blade isn't a "stem". It's a petiole. This may help: http://www.exoticrainforest.com/What is a stem. What is a petiole.html
Technically Ron, that is not completely correct but is found in a variety of sources on the net. The petiole is the stalk that supports the leaf and connects it to the stem. I've been talking to Dr. Croat and four other botanists about this for months. Check any botanical dictionary and you won't find that as a precise definition. The problem appears to be how those sources define the "leaf" and the stalk. The stem supports the petiole which is technically a part of the leaf but the petiole itself is not the support for any individual leaf lamina (blade) and should not be called a "stem". People will continue to say it and as a part of horticulture I'm sure we're stuck with it but it isn't a part of botanical science. My quote from Dr. Croat came just today as a part of this continuing discussion. If you'd like to write a letter disagreeing with these botanists please post it and I'll forward it to them all for their personal comments.
Just to be sure I sent botanist Peter Boyce in Malaysia a quick note and asked "Is there ever a situation where the support for a leaf is correctly known as a "stem"?" Pete was in a hurry to catch a plane to Thailand but responded "The quick answer is never. Very poor use of the word stem." Pete is one of the co-authors of the scientific text The Genera of Araceae published by the Royal Botanic Garden Kew in London. His co-authors are botanists Simon Mayo and Josef Bogner. Dr. Croat again responded "Steve, It is very simple. The stem has leaves which extend outward. These each have petiole Each petiole has a blade." But notice he said each leaf has a petiole and the petiole is the actual stalk that support the leaf blade. The problem is with the way individuals interpret the definition. The stem is not the stalk that supports each leaf but is instead the base axis of the plant composed of nodes and internodes producing roots, petioles and the sexual parts of the plant (flowers). The petiole is the blade support. Botanist Dr. Thomas B. Croat Ph.D., P.A. Schulze Curator of Botany of the Missouri Botanical Garden in St. Louis, MO is one of the world's most respected botanists. Tom has personally collected more than 100,000 wild specimens in the rain forests of the world. He has granted more scientific names to new species than perhaps any other living botanist and has published countless documents and journals. Tom also serves as a professor of Botany at St. Louis University. Once again, this may help anyone to understand. The page includes illustrations. http://www.exoticrainforest.com/What is a stem. What is a petiole.html
